Some thermal treatments are carried out by arranging the parts to be treated in chamber and by injecting gas into this chamber. For example, hardening (gas-cooling) operations are achieved by introducing nitrogen under very high pressure then and then draining this gas off to open air at the end of each cycle.
In some hardening cycles, it is necessary to use more precious gases, such as helium, which permit cooling rates higher than those obtained with nitrogen. In this case, but also in many other treatments using a gas in a treatment chamber, it is necessary to recover the gas extracted from the treatment chamber in order to use it again during the next treatment cycle. The treatment gas is thus recovered for economical reasons if it is expensive, or for safety reasons if this gas is toxic or dangerous.
When a gas under a relatively high pressure has to be injected into a treatment chamber, it is necessary to use a relatively large-size pumping means in order to pump the gas from a storage tank to the treatment chamber. The same pumping means or another large-size pumping means is used to pump the gas out of the treatment chamber to carry it to the storage tank.
In some applications, it is desirable that the time taken for filling the chamber and draining it off be relatively short. In such case, the pumping means used (compressors) have to transfer very high gas flows and bring the gas to a relatively high pressure. On the other hand, the nature of the gases used can be such that pumping presents problems, for example if the gases are corrosive.